The recent spread of portable electronic devices with compact design such as cellular phones and hearing aids is remarkable.
Devices that operate in contact with the skin of a living body also get more varied and spread. For example, a vital sign remote monitoring device that measures or monitors vital signs, such as body temperature, blood pressure, and pulse, and automatically transmits the obtained information to a hospital and the like has been developed. A patch-type medicine supplying device that, upon the application of a potential, supplies medicine through the skin has also been developed.
Under these circumstances, the batteries for supplying electric power to the above-mentioned devices are required to be thinner and more flexible.
Thin batteries that have been already developed include paper batteries, thin flat batteries and plate-like batteries. These batteries have a hard housing and are excellent in strength. However, this feature could be an obstacle to making the battery more flexible and thinner.
In view of this, development has been made to a battery with high flexibility (hereinafter referred to as a “flexible battery”) including a housing made of a thin and flexible film-like material (e.g., a laminated film) (see, e.g., Patent Literatures 1 and 2). These batteries are advantageous also in terms of the energy density. Patent Literature 1 proposes a flexible battery including a sheet-like electrode group obtained by stacking sheet-like positive and negative electrodes with a separator interposed therebetween, and a housing made of a laminated film (hereinafter sometimes referred to as a “laminate housing”) enclosing the electrode group.
However, a laminated film used as a material for a laminate housing usually has a metal layer which serves as a barrier layer, and resin layers which serve as a seal layer and a protective layer. Such a laminated film is low in stretchability. Using it as it is for a housing of a battery will limit the improvement in flexibility of the battery. Moreover, if such a laminate housing with low stretchability is forcibly bent, the housing may rupture or break. The rupture or breakage of the laminate housing, if any, may result in evaporation of the electrolyte or deterioration in battery performance.
Conventionally, in order to improve the flexibility of a flexible battery, a proposal has been made to provide the laminate housing with a corrugated portion (see, e.g., Patent Literature 3).
In the case where the laminate housing is provided with a corrugated portion, when the laminate housing is bent, the outside of the corrugated portion will stretch in the direction along which stress is applied, as the length between one top and the next top of a corrugation becomes larger; while the inside of the corrugated portion will shrink in said direction, as the length between one top and the next top of a corrugation becomes smaller. In this manner, the laminated film can deform following the bending of the laminate housing, and thus the flexibility of the flexible battery is improved.